Additive manufacturing or rapid prototyping methods for producing components comprise layer-by-layer solidification of a material, such as a metal powder material, using a laser beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is scanned across portions of the powder layer that correspond to a cross-section of the component being constructed. The laser beam melts or sinters the powder to form a solidified layer. As explained in more detail below with reference to FIG. 2, it is typical to melt or sinter the desired pattern in the powder layer using a series of stripes. In particular, it is known to advance so-called hatch lines back and forth along a plurality of stripes in turn to construct the desired pattern in the powder layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified as required.
During the melting or sintering process, debris (e.g. condensate, unsolidified particles of powder etc) is produced within the build chamber. It is known to introduce a gas flow through the build chamber in an attempt to remove debris from the chamber in the gas flow. For example, the M270 model of machine produced by EOS GmbH, Munich, Germany, passes a flow of gas from the top of the build chamber towards the powder bed and various exhaust vents collect the gas for recirculation. The gas flow in the M270 machine is thus turbulent and has no well defined flow direction. The newer M280 model of machine produced by EOS comprises a series of gas outlet nozzles located to the rear of the powder bed that pass a flow of gas to a series of exhaust vents that are located at the front of the powder bed. In this manner, a planar layer of gas flow is created at the surface of the powder bed. This planar gas flow arrangement has, however, been found by the present inventors to produce high surface roughness and non-uniformity of the solidified metal layers generated by the melting process.